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U-Pb memory behavior in Chicxulub's peak ring - Applying U-Pb depth profiling to shocked zircon

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Title: U-Pb memory behavior in Chicxulub's peak ring - Applying U-Pb depth profiling to shocked zircon
Authors: Rasmussen, C
Stockli, DF
Ross, CH
Pickersgill, A
Gulick, SP
Schmieder, M
Christeson, GL
Wittmann, A
Kring, DA
Morgan, JV
Item Type: Journal Article
Abstract: The zircon U-Pb system is one of the most robust geochronometers, but during an impact event individual crystals can be affected differently by the passage of the shock wave and impact generated heat. Unraveling the potentially complex thermal history recorded by zircon crystals that experienced variable levels of shock and heating, as well as additioanl pre- and post-impact thermal events, has been difficult using classical geochronological methods. The existing high-precision 40Ar/39Ar age constraints for the K-Pg Chicxulub event, and the previous U-Pb dating of the basement rocks from the impact site, make Chicxulub an ideal location to study impact-induced effects on the zircon U-Pb systematics and to evaluate potential 'memory effects' of pre-impact U-Pb signatures preserved within those individual zircon crystals. Recent IODP-ICDP drilling of the Chicxulub impact structure recovered 580 m of uplifted shocked granitoid and 130 m of melt and suevite, providing an unprecedented opportunity to study zircon crystals subjected to a range of shock pressures, thermal, and deformational histories. Zircon morphologies were classified using scanning electron microscopy (SEM) imaging and then samples were depth profiled using laser ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS) to document the range of preserved age domains from rim-to-center within individual crystals. The results show U-Pb ages range from 66 to 472 Ma, which are consistent with both inherited Carboniferous and Late Paleozoic basement ages as well as Pb loss ages in response to the K-Pg impact event. While the bulk of the zircon grains preserve Paleozoic ages, high U (metamict) zones within fractured zircon crystals exhibited an age within uncertainty (66 ± 6.2 Ma) of the impact age (66.038 ± 0.049 Ma), indicating that inherited intragrain U-Pb kinetics and/or hydrothermal fluid flow may have controlled age resetting those zircon crystals rather than impact-induced shock and heating alone. Moreover, the calculated α-decay doses suggest that the zircon crystals experienced Stage 1 or early Stage 2 radiation damage accumulation. Therefore, we suggest that the lowered crystal annealing temperature in crystals that previoulsy experienced radiation damage make the zircon U-Pb clock either more susceptible to the relatively short heat pulse of the impact event, the moderate pressure and temperature conditions in the peak ring, and/or to hot-fluid flow in the long-lasting post impact hydrothermal system.
Issue Date: 20-Oct-2019
Date of Acceptance: 24-Jul-2019
URI: http://hdl.handle.net/10044/1/77978
DOI: 10.1016/j.chemgeo.2019.07.029
ISSN: 0009-2541
Publisher: Elsevier
Start Page: 356
End Page: 367
Journal / Book Title: Chemical Geology
Volume: 525
Copyright Statement: © 2019 Elsevier B.V. All rights reserved. This manuscript is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International Licence http://creativecommons.org/licenses/by-nc-nd/4.0/
Sponsor/Funder: Natural Environment Research Council (NERC)
Funder's Grant Number: NE/P005217/1
Keywords: Science & Technology
Physical Sciences
Geochemistry & Geophysics
Chicxulub
U-Pb systematics
LA-ICP-MS depth profiling
Metamictization
IODP
RADIATION-DAMAGE
IMPACT CRATER
AGE
LEAD
METAMICTIZATION
GEOCHRONOLOGY
EVENT
SYSTEMATICS
MONAZITE
MELT
Science & Technology
Physical Sciences
Geochemistry & Geophysics
Chicxulub
U-Pb systematics
LA-ICP-MS depth profiling
Metamictization
IODP
RADIATION-DAMAGE
IMPACT CRATER
AGE
LEAD
METAMICTIZATION
GEOCHRONOLOGY
EVENT
SYSTEMATICS
MONAZITE
MELT
Geochemistry & Geophysics
0402 Geochemistry
0403 Geology
0406 Physical Geography and Environmental Geoscience
Publication Status: Published
Online Publication Date: 2019-07-27
Appears in Collections:Earth Science and Engineering